Add -D_XOPEN_SOURCE to the Mac build, because it was failing on a "deprecated" error in the ucontext.h header.
Add -D_XOPEN_SOURCE to the Mac build, because it was failing on a "deprecated"
error in the ucontext.h header.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1017
TEST= try to build sel_ldr on Mac OS X 10.6

Review URL: http://codereview.chromium.org/3745002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@3456 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client